b 1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus in which optical information is recorded on and/or reproduced from an optical information recording medium by illuminating light beams on optical tracks formed on the optical information recording medium via an objective lens. The invention particularly relates to an apparatus having an objective lens position sensor; and an offset of a tracking error signal can be effectively removed on the basis of an output of the objective lens position sensor.
2. Prior Art Statement
Optical discs and optical cards have been suggested as optical information recording media. FIG. 1 is a schematic view showing one example of a construction of the optical card. The optical card 1 comprises a data record region 2 on which optical information is recorded. FIG. 2 is a schematic view showing one part of the data record region 2 formed on the optical card 1. On the data record region 2, are formed a plurality of guide tracks 4, and data tracks 3 are formed therebetween. It should be noted that the tracks 3 and 4 are formed to be parallel to each other; and optical information data is recorded on the data tracks 3.
While, in the optical information recording/ reproducing apparatus, for instance, optical information is recorded on and/or reproduced from the optical information recording medium in such a manner that the optical information recording medium is moved in a tracking direction, which is parallel with the extending direction of the tracks, while light beams for recording and reading optical information are made incident upon the tracks via an objective lens. In such apparatus, as well known, focusing and tracking error signals for correcting the position of the objective lens with respect to the optical information recording medium are produced on the basis of reflection light beams reflected by the optical recording medium; and in order to let the light beams follow the tracks in a focus condition, a focusing servo operation is conducted such that the objective lens is moved in a direction (focusing direction F), which is parallel to the optical axis thereof, by a focusing actuator to which said focusing error signal is supplied, and a tracking servo operation is conducted such that the objective lens is driven in a direction (focusing direction D), which is perpendicular to the optical axis thereof and the track direction, on the basis of the tracking error signal.
FIG. 3 is a schematic view showing one example of a construction of an optical system used in the optical information recording/reproducing apparatus. In this apparatus, the focusing error is detected by the off-axis method and the tracking error by the triplicated beam method. In FIG. 3, a light beam emitted from a semiconductor laser 5 is changed into a parallel light beam by a collimator lens 6; the parallel light beam is separated into three light beams by a diffraction grating 7; and the three light beams are made incident upon the information recording region 2 of the optical card 1 via an objective lens 8. As clear from FIG. 3, the light beams do not travel on an optical axis of the objective lens 8.
FIG. 4 is a schematic view showing light beam spots of the three light beams formed on the tracks on the optical card 1. The light beam spots are formed on the tracks in such a manner that a main beam 9a, which is for recording and/or reproducing optical information, follows a center portion of the data track 3; and subbeams 10a, 11a, which are for detecting the tracking error signal, follow inner edge portions of guide tracks 4,4, in an even manner, respectively. Reflection light beams corresponding to these three beams 9a, 10a and 11a are made incident upon a photo detector 14 via the objective lens 8, a reflection mirror 12 and a lens 13; and then information data signal and error signals are detected by the photodetector 14.
FIG. 5 is a schematic view illustrating light receiving regions of the photo detector 14. The photo detector 14 comprises three light receiving elements 15, 16 and 17. A center element 15 of the light receiving regions is for receiving the reflection light beam 9b of the main beam 9a; and the center light receiving element 15 is further divided into two light receiving regions 15a and 15b. The other side light receiving elements 16 and 17 are for receiving the reflection light beams 10b and 11b of the sub-beams 10a and 11a. The reflection light beams 9b, 10b and 11b being made incident upon the photo detector 14, are moved in a direction shown by an arrow, which is perpendicular to a divisional line of the center light receiving element 15, in accordance with a variation of a relative positional relationship in a focusing direction between the optical card 1 and the objective lens 8. Therefore, a ratio of light amount between the light receiving regions 15a and 15b is varied in accordance with the movement of the reflection light beam 9b, which is moved in the direction shown by the arrow. Additionally, when a relative positional relationship between the optical card 1 and the objective lens 8 in a tracking direction T is varied, the light amounts of the reflection light beams 10b, 11b, which are made incident upon the light receiving regions 16 and 17, are varied.
In the conventional apparatus, the outputs of the light receiving regions 15a and 15b are supplied to a differential amplifier 18 to detect a difference there between, i.e. a focusing error signal, as shown in FIG. 6. When the relative position of the optical card 1 and the objective lens 8 is in a focusing condition, the difference between the outputs of the light receiving regions 15a and 15b becomes zero; thus, it is possible to obtain a focusing error signal, whose polarity is converted in accordance with a direction of the relative position between the optical card 1 and the objective lens 8, from the difference. The thus obtained focusing error signal is supplied to a focusing actuator, which is for driving the objective lens 8 in the focusing direction F, in order to conduct the focus servo control operation.
On the other hand, as shown in FIG. 7, the outputs of the light receiving regions 16 and 17 are supplied into a differential amplifier 19 to detect a difference therebetween. When, the sub-beams 10a and 10b are made incident upon the the guide tracks 4 in an on-track condition, the difference becomes zero. Therefore, a tracking error signal, whose polarity is converted in accordance with the direction that the sub-beams 10a and 10b are deviated from the guide tracks 4, can be obtained from the difference. The thus obtained tracking error signal is supplied to a tracking actuator 23, by which the objective lens 8 is driven in the tracking direction T, via a switch 20, a phase compensator 21 and a driving amplifier 22 in order to carry out the tracking servo control operation.
FIG. 8 is a perspective view depicting an example of a construction of an example of an objective lens driving device, by which the objective lens 8 is driven in the focusing direction F and the tracking direction T. A lens holder 31 is supported on a fixing member 35 to be movable in the focusing and tracking directions by means of a pair of leaf springs 32a, 32b, which are for driving the objective lens 8 in the focusing direction, an intermediate supporting member 33, and a pair of leaf springs 34a, 34b, which are for driving the objective lens 8 in the tracking direction. On both side surfaces of the lens holder 31 in the tracking direction T, are provided focusing coils 36a, 36a, to which the focusing error signal is supplied, and tracking coils 36b, 36b, to which the tracking error signal is supplied, respectively. Further, on the fixing member 35, is arranged a pair of magnetic field generating members 37a, 37b, which comprise permanent magnets for driving the objective lens 8 in the focusing direction F and the tracking direction T in conjunction with said focusing coils 36a and said tracking coils 36b.
Furthermore, there is provided a mirror 38 on an end surface of the lens holder 31. It should be noted that the mirror 38 is arranged in a plane which is determined by the focusing and tracking directions. While, on the fixing member 35 are provided a light source 40 and a photodetector 41 via a supporting member 39 in such a manner that a light beam emitted from the light source 40 is made incident upon the mirror 38 and the reflection light beam is received in the photo detector 41, as shown in FIG. 9A. The photo detector 41 comprises light receiving element 41 which is divided into two regions 41a and 41b, as shown in FIG. 9B. The photo detector 41 is arranged such that when the lens holder 31, i.e. objective lens 8, is positioned in a reference position in the tracking direction T, the reflection light beam reflected by the mirror 38 is made incident upon the light receiving regions 41a and 41b in an even manner; while when the objective lens 8 is moved and deviated from the reference position in the tracking direction T, the ratio of the light amount of the reflecting light beam being made incident upon the light receiving regions 41a and 41b is varied in accordance with the movement of the objective lens 8 in the tracking direction T. To this end, a difference between outputs of the light receiving regions 41a and 41b, i.e. an off-set of the objective lens 8 from the reference position in the tracking direction T, is detected. FIG. 10 is a graph showing a variation of the off-set signal, which is varied in accordance with the movement of the objective lens 8 in the tracking direction T.
In Japanese Preliminarily Patent Publication No. 1-201831, is disclosed a method for detecting the fact that the laser beam cannot trace the track correctly on the basis of the off-set signal; and in Japanese Patent Publication No. 62-15933, is disclosed a technique for detecting whether the objective lens is stopped in a static manner after a desired track is sought in accordance with the off-set signal.
In the conventional optical information recording/reproducing apparatus explained in the above, when the apparatus is set in a horizontal manner, a neutral position of the objective lens 8 is coincident with the reference position in its driving range in the tracking direction T. It should be noted that the neutral position means a position at which the objective lens is positioned when the tracking servo loop of the apparatus is opened. However, the apparatus is sometimes set in an inclined manner actually. When the apparatus is set in an inclined manner, the neutral position of the objective lens 8 is deviated from the reference position. That is to say, a half of the driving range of the objective lens 8 in the tracking direction T becomes small, so that there would occur problems such as an off-set of the tracking error signal and a decrease of a sensitivity of the tracking error signal; and then the tracking servo operation could not be conducted in a stable manner At the worst, the objective lens 8 is moved over the driving range thereof, so that the tracking servo operation could not be conducted at all. Furthermore, it would be impossible to detect the fact on the basis of the off-set signal that the light beam is out of the desired tracks and to detect the fact that the objective lens 8 is stopped in a stable manner after the desired track is sought, due to the non-linear off-set signal.